<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="vue.js" type="text/javascript" charset="utf-8"></script>

</head>
<body>
<div id="app">
    {{ a }}
</div>
<script type="text/javascript">
    var data = {a: 1};
    var vm = new Vue({
        el: '#app',
        data:data
    });
    // data.a = "hi .... new ...."
    // data.a = "test...."
    // vm.$data.a = "@...."
    vm.$watch('a',function (newVal, oldVal) {
        console.log(newVal,oldVal);
    });
    vm.$data.a="test...."
</script>
</body>
</html>